An antenna device for use in a base station for portable telephones and the like comprises an antenna array which is made up of a plurality of antenna elements. A description will be given of the characteristics of radio-waves delivered from an antenna device including a plurality of antenna elements to a terminal station.
FIG. 1 is a perspective view showing an exemplary configuration of an antenna device. Here, as shown in FIG. 1, 13 antenna elements 120 are arranged at equal intervals in a line in a direction perpendicular to the ground. Each of antenna elements 120 is connected to feed circuit 100.
FIG. 2 is a graph showing an example of an amplitude distribution and a phase distribution. The vertical axes of the graph indicate the amplitude on the left scale and the phase on the right scale. The sign of the phase indicates an advanced phase when it is positive, and a delayed phase when it is negative. The horizontal axis of the graph represents numbers of antenna elements 120. This number indicates the location of the antenna element.
The centrally positioned antenna element is given number “0” among 13 antenna elements 120, antenna elements on one side of the antenna element No. 0 are given numbers with plus sign in order, and antenna elements on the other side of the antenna element No. 0 are given numbers with minus sign in order. The antenna elements positioned at both ends in the linearly arranged ones are given number “−6” and “+6.” In the following, the side accompanied with the plus numbers are called the plus side, while the side accompanied with the minus numbers are called the minus side.
The values of amplitude and phase plotted on the graph of FIG. 2 indicate the amplitude and phase of a radio-wave at each antenna element. The amplitude and phase at the central antenna elements are zero, respectively, because they are used as the basis.
As shown in FIG. 2, the amplitude distribution reaches a maximum at antenna element No. 0 at the center, and presents a value which decreases as the absolute value of the number of the antenna element increases. The amplitude distribution exhibits an even function characteristic which has a central axis that passes through antenna element No. 0.
As shown in FIG. 2, the phase distribution exhibits a characteristic which includes a step near the center. The radio-waves are equal in phase at antenna elements +1 through +6, and connecting the values of the respective phases result in a flat distribution without unevenness. The phases of radio-waves at antenna elements of No. −1 through No. −6 also exhibit a flat distribution like the plus number side. The phase distribution presents an odd function characteristic which has its origin at antenna element No. 0.
While FIG. 2 shows a phase distribution which includes a flat portion, the phase distribution may include inclined portions. However, the angles of the slope are the same on the plus side and minus side of the antenna elements. The phase distribution also exhibits an odd function characteristic in this case.
Next, a description will be given of a radiation pattern when radio-waves delivered from an antenna device are combined.
FIG. 3 is a graph showing an example of a radiation pattern for the radio-waves which exhibit the characteristic shown in FIG. 2. The vertical axis of the graph represents the gain, while the horizontal axis represents the angle where the horizontal direction of antenna elements 120 is determined to be at 90 degrees. As shown in FIG. 1, a sky side from antenna element 120 covers a range from 90 degrees to 0 degree on the horizontal axis of the graph, while a ground side from antenna element 120 covers a range from 90 degrees to 180 degrees on the horizontal axis of the graph.
A solid line in FIG. 3 represents a radiation pattern of the antenna which exhibits the characteristic shown in FIG. 2, and a broken line represents an ideal characteristic. The ideal radiation pattern is a cosec square curve. The radiation pattern by the radio-waves with the amplitude distribution and phase distribution shown in FIG. 2 forms a null fill beam. JP2006-197530A (hereinafter, called “Document 1”), for example, discloses the configuration, amplitude distribution, and phase distribution of an antenna which forms a null fill beam.
On the other hand, an example of a method of approaching a radiation pattern to an ideal one is disclosed on Page 117 of “Electromagnetic Wave Engineering” (written by Saburo Adachi, and published by Colona Publishing Co., Ltd in 1983, hereinafter called “Document 2”).